Added support for Unicode file names on MS-Windows.

[Eli Zaretskii]

Starting with trunk revision 115490, Emacs on Windows can access files
whose names include characters outside of the current ANSI codepage.
This has been the subject of several bug reports in the past.

People who are interested in technical details are invited to read the
large commentary around line 1300 in w32.c.  In a nutshell, Emacs on
MS-Windows now pretends that file names are encoded in UTF-8, and does
whatever conversions are necessary in functions that replace or are
wrappers around C library functions which accept file-name arguments.

Please report any bugs these changes introduced.

Many thanks to Ken Brown who generously used some of his time to find
and report my bugs which would have otherwise broken the Cygwin build.

Enjoy.